1. Field of the Invention
The present invention relates generally to communication systems, and more particularly, to a method for forward link inter-generation soft handoff between second generation (2G) and third generation (3G) Code Division Multiple Access (CDMA) systems.
2. Description of the Related Art
One commonly used type of cellular radiotelephone communication system is referred to as a Code Division Multiple Access (CDMA) system. In a CDMA system, the radio signals share the same frequency spectrum at the same time, in contrast to previous Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) systems. One current CDMA standard, known as the second generation standard or 2G, is designated as TIA/EIA-95-A/B (or IS-95-A/B), and is herein incorporated by reference. More recently, a new third generation (3G) CDMA standard has been proposed and has been designated as IS-2000 (previously IS-95-C) or CDMA2000, and is herein incorporated by reference. As the new 3G systems are installed, cellular systems will contain a mix of both old 2G systems and the new 3G compatible systems.
In a typical CDMA cellular radiotelephone communication system, a mobile station communicates with a base station having the strongest available signal. In order to track the available signals, the mobile station maintains a list of available base stations. Specifically, each base station in the CDMA system transmits an unmodulated xe2x80x9cpilotxe2x80x9d signal on a set of predetermined frequencies. A mobile station receives the pilot signals and determines which pilot signals are the strongest. A xe2x80x9csearcherxe2x80x9d unit located in the mobile station commonly performs the signal detection and strength measurement functions.
The results from the searcher are reported to the current (i.e. active) base station. The base station then instructs the mobile station to update a list of available base stations maintained by the mobile station. The list is sub-divided into three operative setsxe2x80x94an active set, a candidate set, and a neighbor set. The active set contains a list of the base stations with which the mobile station is currently communicating (typically 1-4 base stations). The candidate set is a list of base stations which may move into the active set, and the neighbor set is a list of base stations which are being monitored, but less frequently.
As the mobile station moves and its currently active base station signal weakens, the mobile station must access a new base station. Based upon the results of the searcher, and the instructions received back from the base station, the mobile station will update its sets, and communicate with a different base station. In order for communication transmissions to appear seamless to the user of the mobile station, the communication link must be handed off to the next base station. Ideally, this handoff would establish a new link before terminating the first link. This type of handoff is known as a soft handoff (SHO) or xe2x80x9cMake-Before-Break.xe2x80x9d
Presently, a SHO cannot occur between two different generations of CDMA systems. The 3G system has been designed to provide backward compatibility with the 2G system at the signaling and call processing level. However, since these two systems employ different modulation schemes and spreading rates, they are not naturally compatible at the physical layer. Therefore, at the service boundaries between the 2G and 3G systems, a hard handoff, also known as a xe2x80x9cBreak-Before-Makexe2x80x9d method, has been proposed.
In this type of hard handoff, the connection with a currently active base station (i.e., 2G) is terminated before the new service with the new base station (i.e., 3G) is established. This type of service disruption lowers the quality of service (QOS) for the cellular telephone user. In this scenario, if the mobile station is engaged in a voice service, the user will most likely experience unpleasant voice quality degradation or even call drop. If the mobile station is transferring data, significant transmission delays (due to retransmission errors) will likely occur. In fact, the current standard causes a minimum of 10 frames to be lost, before service is restored.
Thus, it would be desirable to provide a soft handoff between two different generations of CDMA systems, in order to avoid the disadvantages associated with the currently proposed hard handoff scheme.
The present invention is a modification to the proposed IS-2000 specification, in order to provide soft handoffs on forward links between two different generations of CDMA systems. In general, the present invention modifies the proposed messaging structure to allow for reporting of the generation type of the base stations. Two different embodiments are disclosed, as well as two possible soft handoff procedures. The present invention is not limited to the disclosed preferred embodiments, however, as those skilled in the art can readily adapt the teachings of the present invention to create other embodiments and applications.
In a first embodiment, a system configuration parameter is added to the General Handoff Direction Message, and Extended Handoff Direction Message. The Neighbor List Message and Extended Neighbor List Message are updated to include information concerning both systems"" base stations. A selection based soft handoff is used to perform the handoff between inter-generation systems. The selection is based on the signal strength of the received pilot signals.
In a second embodiment, four parameters are added to the PILOT_PN record of the General Handoff Direction Message. The four parameters are a generation identification parameter, a radio configuration parameter, a drop timer parameter and a drop threshold parameter. Based upon the values of the generation identification parameter and the radio configuration parameter, either a selection based soft handoff or a true handoff is performed. The true handoff combines signals from both generation systems, before dropping a current base station in favor of the stronger other generation base station. The drop parameters may be used to provide a sufficient time overlap for the two different base stations, and to allow a system designer to tune the network.